It is well known that the Long Term Evolution (referred to as ‘LTE’ hereinafter), so named because it evolutionized the 3rd generation (referred to as ‘3G’ hereinafter) mobile communication in long term perspective, is one of the strong candidates for the 4th generation mobile communication technology in parallel with the Wibro Evolution. This LTE is based on the ‘Release 8’ that is finalized as a standard specification in December 2008 by the 3rd Generation Partnership Project (referred to as ‘3GPP’ hereinafter) which standardizes 3rd mobile wireless communication; the channel bandwidth is from 1.4 MHz to 20 MHz, and the maximum transmission speed of the downlink is 100 Mbps based on 20 MHz bandwidth, and the maximum transmission speed of the uplink is 50 MHz. Wireless multiple access and multiplexing method is based on orthogonal frequency-division multiplexing (referred to as ‘OFDM’ hereinafter), and high speed packet data transmission method is based on multiple-input and multiple-out (MIMO). LTE Advanced is an evolutionized version of the above-described LTE, it will be referred to as ‘3GPP LTE’ hereinafter.
Meanwhile, feed-through of a mixer or carrier leakage caused by the I/Q DC offset, therefore I/Q offset should be eliminated properly because it may function as interference in the system and deteriorate the system performance.
In OFDM, the transmitter does not transmit data to DC(0) subcarrier, as a result, the I/Q offset effect appears at the DC subcarrier in the receiver making measurement and cancellation of the I/Q offset very easy.
Meanwhile, at present time, SC-FDMA is adopted to reduce high peak-to-average power ratio (referred to as ‘PAPR’ hereinafter) that is a disadvantage of OFDM technology in a 3GPP LTE uplink.
FIG. 1 presents an approximate functional block diagram of a transmitter and a receiver of a 3GPP LTE in a conventional uplink, wherein the transmitter functions as a terminal while the receiver operate as a part of the base station. As shown in FIG. 1, a transmitter of a 3GPP LTE in a conventional uplink includes a serial to parallel converter 102, a subcarrier mapping module 106, an M-point inverse discrete Fourier transform (referred to as ‘IDFT’ hereinafter) module 108, a cyclic prefix adding module 110, a parallel to serial converter 112, and a radio frequency (RF)/digital to analog converter (DAC) module 114. Signal processing steps in an orthogonal frequency-division multiple access (referred to as ‘OFDMA’ hereinafter) transmitter can be described as follows. First, a bitstream is converted into a data symbol sequence. Bitstreams can be obtained through various signal processing such as channel encoding, interleaving, or scrambling applied to the data blocks transmitted from the medium access control (MAC) layer. A bitstream, so-called ‘codewords,’ is equivalent to a data block that is transmitted from the MAC layer. Next, such serial data symbol sequence is converted into N parallel data symbols (102). N data symbols are mapped to N sub-carriers that are allocated among the total of M sub-carriers, and the other remaining M-N sub-carriers are padded to 0 (106). The data symbols mapped to frequency domain are converted into a time domain sequence through the M-point inverse discrete Fourier transform (108). Later, OFDMA symbols are generated by adding cyclic prefix to the above-described time domain sequence in order to reduce inter-symbol interference (ISI) and inter-carrier interference (ICI) (110). The generated OFDMA symbols are converted into the serial format from the parallel format (112). Then, the OFDMA symbols are transmitted to the receiver through the processes such as digital-to-analog conversion, and frequency up-conversion (114). Available sub-carriers among the remaining M-N sub-carriers are allocated to the other users.
Next, an OFDMA receiver is comprised of a RF/ADC module 116, a serial-to-parallel converter 118, a cyclic prefix removing module 120, an M-point discrete Fourier transform (referred to as ‘DFT’ hereinafter) module 122, a sub-carrier demapping/equalization module 124, a parallel-to-serial converter 128, and a detection module 130. The signal processing steps of an OFDMA receiver is comprised of the signal processing steps of an OFDMA transmitter in a reverse manner.
Meanwhile, a SC-FDMA transmitter, when compared to the OFDMA transmitter, further includes an N-point DFT module 104 at the front end of the sub-carrier mapping module 106. The SC-FDMA transmitter, when compared to the OFDMA transmitter, can significantly reduce PAPR of the transmitted signal by distributing the multiple data into the frequency domain through the DFT before the IDFT processing. A SC-FDMA receiver, when compared to the OFDMA receiver, further includes an N-point IDFT module 126 added at the back end of the sub-carrier demapping module 124. The signal processing steps of an SC-FDMA receiver is comprised of the signal processing steps of an SC-FDMA transmitter in a reverse manner.
However, unlike OFDM, the transmitter transmits data to DC sub-carrier in a 3GPP LTE uplink. In this case, performance of the receiver is deteriorated due to the distortion of the data near the DC sub-carrier caused by the effect of the I/Q offset in the transmitter. To prevent this problem, in 3GPP LTE, SC-FDMA transmission is performed after shifting the frequency by Δf/2 which corresponds to the half of the sub-carrier spacing Δf, i.e. 15 KHz.
FIG. 2 illustrates a sub-carrier whose frequency is shifted by Δf/2 at the SC-FDMA transmitter. As shown in FIG. 2, if the data is transmitted after shifting frequency by Δf/2 in the transmitter, the I/Q offset appears at the center of each sub-carrier since every sub-carrier frequency is shifted by Δf/2.
However, the sub-carriers whose frequencies are shifted by Δf/2 lost their periodicity, and the energy of each sub-carrier is leaked to all over the frequency band through fast Fourier transform (FFT). Moreover all the sub-carriers having I/Q offset interfere each other due to this energy leakage resulting in a serious performance deterioration because accurate measurement and cancellation of the I/Q offset become impractical.